1. Field of the Invention
The present invention relates to a wire bonding apparatus, and especially to that used for a thin high-density package.
2. Description of the Related Art
The steps for wire-bonding a semiconductor by use of a conventional wire bonding apparatus will be described.
A semiconductor chip is placed on the bed of in a lead frame. A bonding pad is provided on the surface of the semiconductor chip. The position of the bonding pad is assigned as the first bonding position. A region close to the tip end of an inner lead of the lead frame is assigned as the second bonding position.
With the above-described structure, a capillary is conveyed to the first bonding position, One end of the bonding wire is extracted from the capillary, and this end of the bonding wire is electrically connected to the bonding pad at the first bonding position. Next, the capillary is moved in the direction vertical to the surface of the semiconductor chip by the first distance, and then moved in parallel with the surface of the semiconductor chip to the opposite side to the second bonding position by the second distance. After that, the capillary is moved in the direction vertical to the surface of the semiconductor chip by the third distance, and then conveyed to the second bonding position. While the capillary being carried, the trace of the conveyance has an arc shape. At the second bonding position, the other end of the bonding wire is electrically connected to the inner lead.
In the above-described conventional wire bonding device, the loop length defined between the first and second bonding positions is adjusted by the third distance. More specifically, each of the first and second distances is fixed to a certain value, and the loop length is adjusted by changing only the third distance. In the case where the loop length is adjusted in the just-mentioned manner, the relationship between the loop length and the loop height will be as shown in FIG. 1. As can be understood from this figure, the shorter the loop length, the lower the loop height.
Semiconductor devices are different from each other in loop length, and therefore when the semiconductor devices are subjected to wire bonding by use of the conventional wire bonding apparatus, the bonding wires of the devices do not have a constant loop height. In other words, an optimum loop height cannot be achieved in each bonding wire. Especially, in the case of a package with a great number of bonding wires and strict limitations to loop heights, such as a thin and high-density package, the wire bonding operation by use of the conventional wire bonding apparatus entails a problem as below.
In bed-bonding, in which a semiconductor chip is wire-bonded to a bed in a lead frame, since the height of the bed is lower than that of the inner lead, the loop height of this bonding wire is significantly lower than those of the other bonding wires. More specifically, the second bonding positions located near the tip ends of the inner leads are not accurately leveled with each other. Therefore, when a bonding operation is carried out for a bonding wire at its second bonding position, and this second bonding position is located lower than other second bonding positions, this bonding wire will be leveled lower than the other ones by the loop height. As a result, it is likely that an edge touch error, in which the bonding wire and the edge of the semiconductor chip touch with each other, or a bonding error caused by damages in the neck portion of the bonding wire, will occur.